As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries which exhibit high energy density and a high operation potential, long lifespan and low self-discharge ratio are commercially available and widely used.
In addition, as interest in environmental problems is recently increasing, research into electric vehicles (EVs), hybrid EVs (HEVs), and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is actively underway. As a power source of EVs, HEVs, and the like, a nickel metal-hydride secondary battery is mainly used. However, research into lithium secondary batteries having high energy density, high discharge voltage and output stability is actively underway and some lithium secondary batteries are commercially available.
In general, lithium secondary batteries may be classified into lithium-ion batteries containing liquid electrolytes per se, lithium-ion polymer batteries containing liquid electrolytes in the form of gels, and lithium polymer batteries containing solid electrolytes, depending upon types of electrolytes. Particularly, the lithium-ion polymer batteries (or gel polymer batteries) have various advantages such as high safety due to lower probability of fluid leakage as compared to liquid electrolyte batteries, and feasible ultra-thinning and compactness of the battery shape and substantial weight reduction of the battery, which thereby lead to increased demands thereof.
Lithium ion batteries are prepared by impregnating an electrode assembly including a porous separator disposed between a positive electrode and a negative electrode, each of which is coated with an active material, on an electrode current collector in a liquid electrolyte solution including a lithium salt.
Meanwhile, methods of manufacturing lithium ion polymer batteries are broadly classified into a method of manufacturing a non-crosslinked polymer battery and a method of manufacturing a directly-crosslinked polymer battery, depending upon kinds of matrix material for electrolyte impregnation. As the polymer matrix material, acrylate- and methacrylate-based materials having excellent radical polymerization reactivity, and ether-based materials having superior electrical conductivity are mainly used. In particular, the latter directly-crosslinked polymer battery is manufactured by placing a jelly-roll type or stack type electrode assembly composed of electrode plates and a porous separator in a pouch, injecting a thermally polymerizable polyethylene oxide (PEO) based monomer or oligomer crosslinking agent and an electrolyte composition thereto, and thermally curing the injected materials. The battery has advantages in manufacturing processes in that plates and separators of conventional lithium ion batteries can be directly employed without particular modification or alteration. However, this method is known to suffer from disadvantages in that, when the crosslinking agent is not cured and thus remains in the electrolyte, it is difficult to achieve uniform impregnation due to increased viscosity, thereby significantly decreasing characteristics of the battery.
Therefore, there is an urgent need for technology to secure stability of a battery by resolving the problems while maintaining overall battery performance.